1. Field of the Invention
In one aspect, the present invention is related to aqueous acidic compositions for cleaning metal surfaces, and in particular, to aqueous acidic solutions for cleaning aluminum and aluminum alloys.
2. Background Art
Containers comprised of aluminum and alloys thereof are produced in a drawing and forming operation, referred to as drawing and ironing, which results in the deposition of lubricants and forming oils on the surface. In addition, residual aluminum fines, i.e. small particles of aluminum, are deposited on the interior and exterior surfaces of the container during the forming operation. Ordinarily, the exterior surface of the container will have smaller quantities of aluminum fines since during the drawing and ironing step some fines are removed from the exterior surface.
Prior to any processing steps, such as conversion coating and sanitary lacquer deposition, the surfaces of the aluminum containers must be clean and water-break-free so that there are no contaminants which prevent further processing and which render the containers unacceptable for use. “Water-breaks” are understood in the art to be indicative of a contaminated surface.
Acid cleaners have been employed to clean the aluminum surfaces and to remove aluminum fines deposited on the interior walls of aluminum containers. Acid cleaning is ordinarily accomplished at temperatures from 130° F. to 160° F. in order to remove or dissolve the aluminum fines and to remove the lubricants and forming oils so that the surface is rendered water-break-free. The cleanliness of the aluminum surface is measured by the ability of the interior and exterior surfaces of the formed aluminum container to support a continuous film of water that shows no breaks or discontinuities in the film, that is to be water-break-free.
Chromic acid or salts thereof have been utilized in can cleaning technologies to minimize the corrosion of processing equipment by inhibiting the corrosive attack of the acid cleaning composition on the processing equipment. An important shortcoming which cleaners of this kind possess is the inherent toxicity of the hexavalent and trivalent chromium compounds contained therein and the resultant waste disposal problem created by the presence of chromium in the cleaner effluent.
Several prior art metal cleaning compositions contain nonylphenols and rosin ethoxylates. Both of these chemicals have recently come under governmental scrutiny and are regulated in several countries. Nonylphenols are suspected of being endocrine disruptors and rosin ethoxylates are thought to have poor biodegradability. Moreover, high performance cleaners that include rosin ethoxylates tend to be somewhat expensive. Other acidic cleaners are known which omit chromates, nonylphenols, and rosins, but fall short in detergency, stability of the cleaner concentrate and/or are excessively foaming. Accordingly, there exists a need in the prior art for an improved low cost cleaning composition that is stable, safe, low foaming, and has improved biodegradability.